Update wizard_step template with form override

Make it so that any template extending wizard_step can override the form
rendered at the bottom of the page. We'll need this as we transition
from the existing design of the appointment pages to the workflow design
that currently exists in the prototype.

Author: malcolmbaig

manage_breast_screening/core/jinja2/wizard_step.jinja

@@ -31,25 +31,27 @@
 
   {% block step_content %}{% endblock %}
 
-  {% if form %}
-  <form action="{{request.path}}" method="POST">
-    <div class="nhsuk-grid-row">
-        <div class="nhsuk-grid-column-two-thirds">
-          {% block form %}{% endblock form %}
-
-          {% if form.decision %}
-            {{ form.decision.as_field_group() }}
-          {% endif %}
-
-          {{ button({
-            "text": "Continue"
-          }) }}
-
-          {{csrf_input}}
-        </div>
-    </div>
-  </form>
-  {% endif %}
+  {% block wizard_form %}
+    {% if form %}
+    <form action="{{request.path}}" method="POST">
+      <div class="nhsuk-grid-row">
+          <div class="nhsuk-grid-column-two-thirds">
+            {% block form %}{% endblock form %}
+
+            {% if form.decision %}
+              {{ form.decision.as_field_group() }}
+            {% endif %}
+
+            {{ button({
+              "text": "Continue"
+            }) }}
+
+            {{csrf_input}}
+          </div>
+      </div>
+    </form>
+    {% endif %}
+  {% endblock wizard_form %}
 
   {% if cannot_continue_link %}
     <p><a class="nhsuk-link nhsuk-link--no-visited-state" href="{{ cannot_continue_link.href }}">{{ cannot_continue_link.text }}</a></p>